1. Field of the Invention
The present invention relates to a two-stroke engine designed to feed fresh air through a scavenging port formed in a cylinder head and discharge a combustion gas through an exhaust port formed in a lower part of a cylinder to be openable and closable in accordance with a piston.
2. Description of the Related Art
A conventional Schnurle (loop) scavenging type two-stroke engine has fuel consumption and emission problems stemming from air-fuel mixture short circuit. A uniflow type two-stroke engine has been known to overcome this problem.
For example, Japanese Unexamined Patent Application Publication No. 6-288238 discloses a downward uniflow type two-stroke engine in which an exhaust port is formed in a lower portion of a cylinder to open and close at, before or after a piston reaches a bottom dead center, and in which a scavenging port is formed in a cylinder head to be opened as a scavenging valve.
However, this conventional technique is designed to accomplish gas exchange simply by establishing a downward scavenging flow in the cylinder, resulting in a lowering of scavenging efficiency because combustion gas remains at upper corner portions of the cylinder. In addition, difficulty is experienced in stratifying an air-fuel mixture in the case of such a downward scavenging flow, and particularly at low-load operations, a problem arises in that the combustion efficiency is lowered.
Furthermore, the two-stroke engine disclosed in the aforesaid Japanese Unexamined Patent Application Publication No. 6-288238 is designed to drive the scavenging valve by an internal pressure of a crank case, which makes it difficult to properly set a scavenging timing in accordance with an engine operating condition for control of the scavenging flow.
Accordingly, it is an object of the present invention to provide an improved two-stroke engine capable of an efficient combustion at low-load operations.
Another object of the invention is to provide a two-stroke engine capable of easily controlling a scavenging flow to be supplied to a combustion chamber and a scavenging timing.
In accordance with the present invention, there is provided a two-stroke engine having a cylinder block defining a cylinder inside, a piston reciprocally moving within the cylinder and a cylinder head defining a combustion chamber with the piston, comprising a scavenging port formed in the cylinder head to be opened to the combustion chamber at one side with respect to a reference plane passing through a center axis of the cylinder for generating a tumble flow toward a top surface of the piston, and an exhaust port formed in the cylinder block to be opened to the cylinder at the other side with respect to the reference plane in accordance with the reciprocal movement of the piston.
Other features and advantages of the present invention will become more sufficiently understood from the following detailed description.